The present invention relates to an EGR, that is, an exhaust gas recirculation control system for properly re-circulating exhaust gases of an internal combustion engine to an air intake system, a method for obtaining flow quantity of the exhaust gases, and an exhaust gas flow measurement apparatus.
In recent years, an exhaust gas recirculation (EGR) for realizing reduction in NOx by re-circulating part of the exhaust to an air intake system is being widely used. A recirculation quantity (EGR amount) has to be controlled with high accuracy so as to also achieve stability of combustion. In particular, in the case of applying the EGR to a diesel engine, when the EGR becomes excessive, a large quantity of smoke is produced. Consequently, high-accuracy EGR control according to the operating conditions of the engine is necessary.
One of conventional EGR controls is disclosed in Japanese publication No. 2003-516496. The conventional technique is comprised of an intake air mass flowmeter for measuring an intake quantity of air taken into an intake passage, and a recirculation exhaust gas mass flowmeter for measuring an EGR gas quantity provided in an exhaust recirculation passage. The EGR control is performed on the basis of an EGR rate (=EGR gas quantity/(intake air flow quantity+EGR gas flow quantity)) from the intake air flow quantity and the EGR gas flow quantity measured by the respective flowmeters.
In such a conventional technique, high accuracy EGR control according to the operating conditions of the engine cannot be realized sufficiently. Because, in an internal combustion engine, although a quantity of oxygen contained in exhaust gases largely changes according to engine conditions, the conventional EGR system doesn't make special consideration in this respect. That is, in low or intermediate load, a large amount of oxygen is contained in the exhaust gases due to lean-burn operation mode with rich air in mixture, on the other hand, in high load, the oxygen in the exhaust gases decreases due to stoichiometric mixture ratio or rich-burn operation mode.
In the conventional technique, only information of an EGR gas quantity is obtained from the recirculation exhaust gas flowmeter but information of the oxygen in the exhaust gases cannot be obtained. Consequently, it is difficult to perform high-accuracy EGR control.
On the other hand, Japanese publication No. H9(1997)-126060 describes a conventional exhaust recirculation control method of performing EGR control in order to reduce smoke in accordance with changes in the absolute value of oxygen in the total intake air containing EGR gases and in order to reduce NOx in accordance with changes in the concentration of oxygen in the total intake air.
In the conventional technique, at the time of performing the exhaust gas recirculation control by adjusting the opening of the EGR valve in accordance with operating conditions of the engine, oxygen concentration in the total intake air including the EGR gases and the ratio between the quantity of oxygen in the total intake air and the fuel injection quantity (namely oxygen/fuel ratio) is obtained. The oxygen/fuel ratio and a smoke allowable limit value are compared with each other. When the oxygen/fuel ratio is low, the EGR valve is controlled so that the oxygen/fuel ratio becomes equal to the smoke allowable limit value. When the oxygen/fuel ratio is high, the EGR valve is controlled so that the oxygen concentration becomes equal to target oxygen concentration. By the control, an oxygen quantity necessary to maintain the target smoke level can be assured. While always suppressing the smoke exhaust quantity to be within the allowable value, NOx can be reduced maximally.
In this conventional EGR control system, in order to measure the oxygen concentration in the total intake air including the EGR gases and the oxygen quantity, the following techniques is adopted. On the downstream side (engine side) of the position of connection for the exhaust recirculation passage in the intake air passage of an internal combustion engine, a flow measurement means is provided for measuring the flow quantity of a total intake air including EGR gases. The flow measurement means is comprised of a pressure sensor for sensing intake pressure and an intake temperature sensor for measuring intake temperature. An O2 sensor for measuring oxygen concentration in the total intake air is also provided on such a downstream side. The O2 sensor senses the oxygen concentration by using an electromotive force of a solid electrolyte or the like.
The conventional technique constructed as described above has a problem such that actual O2 concentration in the total intake air containing the EGR gas and O2 concentration measured by the O2 sensor are different from each other due to a chemical reaction delay in the O2 sensor. Therefore, the control accuracy, particularly, control accuracy at the time of transition largely deteriorates. In case where the flowmeter for measuring the flow quantity of the total intake air is comprised of the intake pressure sensor and the intake temperature sensor, it is difficult to accurately measure mass flow of the total intake air. Because, although the ratio of gas components in the total intake air containing the EGR gas change according to engine operating conditions, the aforementioned flowsensor cannot sense such changes of the gas components.